The present invention relates in general to a secondary battery protection circuit device, and more particularly to a circuit device for protecting a secondary battery from overcharge and (or) overdischarge, and further to a battery pack having such a secondary battery protection circuit device and a secondary battery.
As for an example of a secondary battery employed in a portable electronic apparatus, there is well known a Li ion secondary battery. Since the Li ion secondary battery contains therein a combustible electrolyte, if the secondary battery undergoes an overcharge by mistake, then a metal Li is separated, which may result in an accident such as an explosion occurring in some cases. In addition, if the secondary battery undergoes an overcharge by mistake, then the usable number of times of repetitive charges/discharges will be remarkably decreased. For this reason, there is employed a protection switch including a power MOSFET and the like for disconnecting a secondary battery from the electronic apparatus when detecting an overcharge or overdischarge. An example of a Li ion secondary battery including such a protection switch is described in "NIKKEI ELECTRONICS" issued by Nikkei BP Company, Nov. 20, 1995, pp.100 to 117.
When a Li ion secondary battery undergoes an overcharge due to some cause or other such as a design mistake or malperformance of a charger, a power MOSFET in the protection circuit included in the battery pack operates so as to disconnect the secondary battery from the charger. However, for a user who is not aware of the function of such a secondary battery pack at all, there arises a problem that when connecting an electronic apparatus as a load to the secondary battery pack in order to operate the electronic apparatus, since the above-mentioned MOSFET remains in an off state, a sufficient voltage and current will not be obtained so that the user misunderstands the battery pack just as if the performance of the battery pack itself is degraded. This results in a reduction of the convenience of the battery pack in use.